1. Field of the Invention
The present invention relates to a data communication apparatus for communicating data, particularly image data.
2. Related Background Art
A facsimile device has been known as one of such apparatus. In the prior art facsimile device, a communication standard such as a G3 standard has been defined by CCITT and a communication sequence is conducted in accordance with the communication standard to communicate image data.
A transmitter which meets G3 standard with ECM (error correction communication mode) is first explained.
When the transmitter receives nsf (non-standard facilities), csi (called subscriber identification) and dis (digital identification signal) signals by a V21 modem (300 bps) of a receiver, it transmits nss (non-standard facilities set-up), tsi (transmitting subscriber identification) and dcs (digital command signal) signals by a V21 modem in the transmitter, followed by TRAINING and TCF (training check) by a high speed modem (9600 bps, 7200 bps, 4800 bps and 2400 bps). After the transmission of TRAINING and TCF by the high speed modem, the transmitter receives responses by the V21 modem. When the transmitter receives fit (failure to train) as the response, it retransmits a signal such as dcs with a lower designation for a high speed modem speed and repeats the above procedure.
When the transmitter receives cfr (confirmation to receive) as the response to TRAINING and TCF, it successively transmits TRAINING, HDLC flag (01111110) pattern (preamble) of approximately 200 ms, HDLC framed image signal and three RCP signals by the high speed modem at the preset transmission rate.
After the transmission of the signals by the high speed modem, the transmitter sends a control signal pps-q ("pps" represents partial (or virtual) page signal and "q" represents eom, eop or mps) by the V21 modem. If the image signal is a partial page signal, the control signal pps-q is pps-null, if the image signal is an end page signal, it is pps-eop (end of procedure), if there is a next document and a mode change is required, it is pps-eom (end of message), and if no mode change is required, it is pps-mps (multi-page signal).
After the transmission of the control signal pps-q, the transmitter receives a response by the V21 modem. If the transmitter receives ppr (partial page request) as the response, it retransmits the frame requested by the ppr by the high speed modem and continues the procedure.
If the transmitter receives mcf (message confirmation) as the response, it transmits TRAINING and the preamble followed by a partial page image signal by the high speed modem if the pps-q signal previously transmitted is pps-null. If it is pps-eop, the transmitter transmits a dcn signal by the V21 modem and then opens the line to terminate the communication. If it is pps-eom, the transmitter returns to a phase B to receive dis by the V21 modem. If it is pps-mps, the transmitter transmits TRAINING and the preamble followed by a next page image signal by the high speed modem. The preamble period of the signal by the V21 modem is one second.
A receiver which meets a G3standard with an ECM function is now explained.
When the receiver is called, it transmits CED followed by nsf, csi and dis by the V21 modem. After the transmission of the signals including dis, the receiver receives nss, tsi and dcs signals by the V21 modem. After the reception of the signals including dcs, the receiver receives a TCF signal by the high speed modem in accordance with the speed information in the dcs signal. If the receiver does not correctly receive TCF, it transmits ftt by the V21 modem and then receives the signals including dcs again by the V21 modem. If the receiver correctly receives TCF, it transmits cfr by the V21 modem and receives an image signal by the high speed modem having the same rate as that of TCF.
After the reception of the image signal, the receiver receives at least one RCP continuously by the high speed modem and escapes from the high speed signal reception mode. After the reception of the high speed signal, the receiver receives pps-q by the V21 modem. After the reception of the pps-q signal, the receiver transmits a ppr signal to request to the transmitter the retransmission of an error frame if the error frame is included in the image signal received immediately previously, and it is set to the reception mode of the image signal by the modem having the same rate as that of the immediately previous image signal. If no error frame is included in the image signal received immediately previously, the receiver transmits an mcf signal by the V21 modem and conducts the following operation in accordance with the pps-q signal received immediately previously.
If the pps-q signal is pps-null, the receiver receives the image signal of the next partial page by the modem having the same rate as that of the immediately previous image signal and continues the sequence.
If the pps-q signal is pps-eom, the receiver returns to the phase B after T2 (six seconds) period and transmits the signals including dis by the V21 modem.
If the pps-q signal is pps-mps, the receiver receives the image signal of the next page by the modem having the same rates as that of the immediately previous image signal and continues the sequence.
If the pps-q signal is pps-eop, the receiver receives a dcn (disconnect) signal by the V21 modem and opens the line to terminate the communication.
The preamble period of the signal by the V21 modem is one second. FIG. 19 shows the normal communication sequence described above.
The improvement of quality of the public network since the basic communication protocol described above has been defined is remarkable and the existing communication protocol includes much waste for many currently available networks and takes a long communication time.
A recent G3facsimile device includes many added functions, and nss and dcs are very long to attain those functions. As a result, the communication time is long.
1. Too much time is required to transmit nss and dcs by the V21 modem.
2. It is almost unnecessary to confirm the line status by TCF and cfr.
3. It is not necessary to transmit three RCP's.
4. It is not necessary to transmit the pps-q signal by the V21 modem.
Facsimile devices having the ECM function are disclosed in U.S. Pat. No. 5,031,179 (issued on Jul. 9, 1991), U.S. Pat. No. 5,075,783 (issued on Dec. 24, 1991) and U.S. patent application Ser. No. 371,847 (filed on Jun. 27, 1989), abandoned. Technique to reduce the communication protocol time is disclosed in U.S. Pat. No. 4,910,506 (issued on Mar. 20, 1991), which reduces the time by eliminating the communication of the protocol signal.